Conventional techniques for measuring liquids, such as fuel, contained in storage tanks under normal gravity forces, are no longer satisfactory under the environmental conditions of space flight wherein low gravity conditions are encountered. Despite extensive research and development in this area, since at least the early 1960's, a satisfactory system for measuring liquid volume and/or mass in space conditions has not been found.
Theoretical solutions to the problem abound. Chief among these proposed solutions are the following:
1. Capacitance gauge units, in which a three dimensional array of wires are strung across the interior of the fuel tank to form sets of capacitance grids. These sensors have been found to be excessively heavy and attitude sensitive.
2. Light attenuation, in which suitable dyes are applied to the fuel to enable light attenuation through the liquid and vapor to determine mass quantity. Suitable dyes were not found and the maintenance of optical parts in the cryogenic environment of certain space fuels was found to be too difficult to warrant further investigation.
3. Nuclear detectors, in which emissions from radioactive sources placed in the fuel tank are used to measure fuel quantity. These detectors were found to be of limited applicability and to present a radiation hazard.
4. PVT gauges in which a diaphragm is moved to produce pressure changes in a reference cavity and the pressure response from pressure transducers in the fuel tank and reference cavity is used to measure liquid volume. The power requirements for compression of large volumes of gas was found to be so excessive as to render this technique impractical.
Other techniques found wanting in certain respects involved resonance infrasonics (weighted externally driven diaphragm which oscillates as a function of ullage compressibility); mass metering (accurate inflow/outflow metering with careful accounting using Corioles flow meters or radio frequency flow meters); and ultrasonic waveguide or acoustic devices.
A technique which appeared to have great promise is described in U.S. Pat. No. 3,540,275 entitled "Method and Apparatus for Measuring Liquid Volume in a Tank" issued Nov. 17, 1970 to Post et al. In the apparatus of the '275 patent, a tank containing a lossy dielectric liquid fuel is excited by electromagnetic energy. The electromagnetic energy is swept or varied over a predetermined frequency band to excite a plurality of modes of oscillation. The number of modes excited is related to the average dielectric constant of the total contents of the tank cavity. From the average dielectric constant value, the mass of the liquid is obtained. The advantages of this system include the use of a single transmitter receiver with direct mass measurement and its applicability to both liquid oxygen (LO.sub.2) and liquid helium (LH.sub.2).
The disadvantages are that the complex relationship between the number of modes excited and the average dielectric constant prevents accuracy of measurement. In practice, the requisite degree of accuracy, i.e., .+-.1%, has not been achieved by the mode counting technique with .+-.8% being the norm, despite many years of testing.
Another prior art system is described in the '275 patent; the swept frequency oscillator approach of U.S. Pat. No. 3,312,107 issued Apr. 4, 1967 to Burns, et al. In the system of the '107 patent, electromagnetic energy from a sweep oscillator is coupled to a tank and a detector determines the resonant frequency of the tank as the energy is swept in frequency. The sweep is interrupted and started over again each time a resonant frequency is detected. This cycle is continuously repeated and the time intervals between sweep cycles, i.e., change in resonant frequency, is used to indicate the quantity of liquid in the tank. Post et al. in the '275 patent criticizes the Burns et al. technique on the grounds that the resonant frequency changes with fuel (liquid) orientation and, hence, is inherently inaccurate.